Grain oriented electrical steel sheets-mainly used as iron cores of transformers are required to have excellent magnetic properties, in particular, less iron loss. To meet this requirement, it is important that secondary recrystallized grains are highly aligned in the steel sheet in the (110)[001] orientation (or so-called Goss orientation) and impurities in the product steel sheet are reduced. However, there are limitations in controlling crystal orientation and reducing impurities in terms of balancing with manufacturing cost, and so on. Therefore, some techniques have been developed to introduce non-uniform strain to the surfaces of a steel sheet in a physical manner and reduce the magnetic domain width for less iron loss, namely, magnetic domain refining techniques.
For example, JP 57-002252 B proposes a technique for reducing iron loss of a steel sheet by irradiating a final product steel sheet with a laser, introducing a high dislocation density region to the surface layer of the steel sheet and reducing the magnetic domain width. In addition, JP 62-053579 B proposes a technique for refining magnetic domains by forming grooves having a depth of more than 5 μm on the base iron portion of a steel sheet after final annealing at a load of 882 to 2156 MPa (90 to 220 kgf/mm2), and then subjecting the steel sheet to heat treatment at a temperature of 750° C. or higher. Further, JP 7-268474 A discloses a technique for providing a steel sheet that has linear grooves extending in a direction almost orthogonal to the rolling direction of steel sheet on a surface of the iron base, and also has continuous crystalline grain boundaries or fine crystalline grain regions of 1 mm or less grain size from the bottom of the linear grooves to the other surface of the base iron in the sheet thickness direction. With the development of the above-described magnetic domain refining techniques, grain oriented electrical steel sheets having good iron loss properties may be obtained.
However, the above-mentioned techniques for performing magnetic domain refining treatment by forming grooves have a smaller effect on reducing iron loss compared to other magnetic domain refining techniques for introducing high dislocation density regions by laser irradiation and so on. The above-mentioned techniques also have a problem that there is little improvement in iron loss of an actual transformer assembled, even though iron loss is reduced by magnetic domain refinement. That is, these techniques provide an extremely poor building factor (BF).
It could therefore be helpful to provide a grain oriented electrical steel sheet that may further reduce iron loss of a material with grooves formed thereon for magnetic domain refinement and exhibit excellent low iron loss properties when assembled as an actual transformer, along with an advantageous method for manufacturing the same.